Method of producing an alkali-free glass

ABSTRACT

An alkali-free glass applicable to a light transparent glass substrate in a liquid crystal display essentially consists, by weight, of basic elements of 40-70% SiO 2 , 6-25% Al 2 O 3 , 5-20% B 2 O 3 , 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO, and a fining agent of a combination of 0.05-3% Sb 2 O 3  and at least one of 0.05-2% SnO 2  and 0.005-1% Cl 2 , which fining agent makes the resultant glass free from bubbles without the use of toxic As 2 O 3  which has been known as the fining agent in the art.

This is a divisional of copending application Ser. No. 08/911,945, filedon Aug. 20, 1997, and which designated the U.S.

BACKGROUND OF THE INVENTION

This invention relates to an alkali-free glass and, in particular, to analkali-free glass for use as a light transparent glass substrate for aliquid crystal display and the like as well as a method of producing thesame.

Conventionally, an alkali-free glass has been used as the lighttransparent glass substrate for the liquid crystal display and the like.The alkali-free glass for use in such a display is required to be freefrom bubbles such as blisters and seeds resulting in a display defect,in addition to various characteristics such as heat resistance andchemical resistance.

To meet such a demand, various kinds of alkali-free glasses have beenproposed. U.S. Pat. No. 4,994,415 discloses an SiO₂—Al₂O₃—B₂O₃—CaO—BaOas one of the alkali-free glasses.

In order to obtain a glass without bubbles, it is important to select afining agent capable of generating a fining gas both in a relativelylow-temperature range in which batch decomposition and degassing isstarted and in a relatively high-temperature range in which fining andhomogenization of a glass are caused. Specifically, the fining agentserves to expel a gas generating during the batch decomposition anddegassing of a glass material, and to enlarge very small bubblesremaining in the glass melt during fining and homogenization processes,whereby enlarged bubbles float up in the glass melt and leave the glassmelt.

In the meanwhile, the alkali-free glass for use as a glass substrate fora liquid crystal display is high in viscosity of the glass melt so thata melting process is carried out at a high temperature in comparisonwith a glass containing an alkali component. In the alkali-free glass ofthe type, the batch decomposition and degassing generally occurs at1200-1300° C. while the fining and the homogenization are performed at ahigh temperature of 1400° C. or more. Under the circumstances, thefining agent is required to produce the fining gas in a wide temperaturerange (on the order of 1200-1600° C.). Presently, As₂O₃ is widely usedas the fining agent.

However, As₂O₃ is highly toxic and may possibly cause environmentalpollution during a manufacturing process of the glass and duringdisposal of a waste glass. In this reason, the use of As₂O₃ is beinglimited.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alkali-free glasswhich does not use As₂O₃ as a fining agent and which is free frombubbles resulting in a display defect, and to provide a method ofproducing the same.

As a result of various experiments, the present inventors have found thefact that above-mentioned object is achieved by using a combination ofSb₂O₃ and at least one of SnO₂ and chloride, as a fining agent insteadof As₂O₃.

According to the present invention, there is provided an alkali-freeglass essentially consisting of basic elements of 40-70% SiO₂, 6-25%Al₂O₃, 5-20% B₂O₃, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10%ZnO, and a fining agent of a combination of 0.05-3% Sb₂O₃ and at leastone of 0.05-2% SnO₂ and 0.005-1% Cl₂, on the base of the weight percent.

According to the present invention, there is also provided a method ofproducing an alkali-free glass having a basic composition essentiallyconsisting by weight of 40-70% SiO₂, 6-25% Al₂O₃, 5-20% B₂O₃, 0-10% MgO,0-15% CaO, 0-30% BaO, 0-10% SrO, and 0-10% ZnO, by preparing a glassbatch having the basic composition free from alkali metal oxide, meltingthe glass batch, and forming the glass melt, which is characterized byadding a combination of 0.05-3 wt % Sb₂O₃ and at least one of 0.05-2 wt% SnO₂ and 0.01-2 wt % chloride in terms of Cl₂ as a fining agent intothe glass batch.

Sb₂O₃ and SnO₂ used in the present invention generate a large amount offining gas (oxygen gas) as a result of chemical reaction followingvariation in valence of Sb ions and Sn ions. Specifically, Sb₂O₃(trivalent) is at first changed to Sb₂O₅ (pentavalent) in alow-temperature range on the order of several hundred degree in ° C. andthen returns to Sb₂O₃ (trivalent) around 1200-1300° C. At this time, alarge amount of fining gas is released. Likewise, when SnO₂(quadrivalent) is changed to SnO (bivalent) at 1400° C. or more, a largeamount of fining gas is released. On the other hand, chloride isdecomposed and volatilized in a temperature range not lower than 1200°C. to generate the fining gas (for example, chroline gas). Particularly,decomposition and volatilization are very active at a high-temperaturerange not lower than 1400° C. to generate a large amount of fining gas.

Therefore, the use of a combination of Sb₂O₃ and at least one of SnO₂and chloride as the fining agent provides a high fining effect at a widetemperature range from a comparatively low temperature for the batchdecomposition and degassing to a high temperature for fining andhomogenizing. Thus, it is possible to provide the alkali-free glasswithout bubbles resulting in a display defect.

DESCRIPTION OF PREFERRED EMBODIMENTS

Next, description will be made about a method of producing of analkali-free glass according to the present invention.

At first, a raw glass mixture or a glass batch is prepared so as toobtain a glass having the above-mentioned composition. Description willhereafter be made about the content of each component in the compositionand the reason why the content is so defined.

SiO₂ is a component serving as a network of the glass. The content ofSiO₂ is 40-70%, preferably, 45-65%. When the content is less than 40%,chemical resistance is degraded and a strain point of the glass islowered so that heat resistance is degraded. When the content is morethan 70%, high-temperature viscosity is increased so that themeltability is deteriorated and that devitrified substances ofcristobalite readily precipitate.

Al₂O₃ is a component to improve the heat resistance and devitrificationresistance of the glass. The content of Al₂O₃ is 6-25%, preferably,10-20%. When the content of Al₂O₃ is less than 6%, the devitrificationtemperature remarkably rises so that the devitrification is readilycaused to occur in the glass. When the content is more than 25%, acidresistance, more particularly, buffered-hydrofluoric-acid resistance isdegraded so that the cloudness is readily caused to occur on the surfaceof a glass substrate.

B₂O₃ is a component serving as a flux to lower the viscosity and tofacilitate melting of the glass. The content of B₂O₃ is 5-20%,preferably, 6-15%. When the content of B₂O₃ is less than 5%, the effectas the flux is insufficient. When the content is more than 20%, thehydrochloric acid resistance is degraded and the strain point is loweredso that the heat resistance is degraded.

MgO is a component to decreasing the high-temperature viscosity withoutlowering the strain point so as to facilitate melting of the glass. Thecontent of MgO is 0-10%, preferably, 0-7%. When the content of MgO ismore than 10%, the buffered-hydrofluoric-acid resistance of the glass isseriously degraded.

CaO has a function similar to MgO. The content of CaO is 0-15%,preferably, 0-10%. When the content of CaO is more than 15%, thebuffered-hydrofluoric-acid resistance of the glass is seriouslydegraded.

BaO is a component to improve the chemical resistance and thedevitrification resistance of the glass. The content of BaO is 0-30%,preferably, 0-20%. When the content of BaO is more than 30%, the strainpoint is lowered so that the heat resistance is degraded.

SrO has an effect similar to BaO. The content of SrO is 0-10%,preferably, 0-7%. The content of SrO more than 10% is unfavorablebecause the devitrification is increased.

ZnO is a component to improve the buffered-hydrofluoric-acid resistanceand the devitrification resistance. The content is 0-10%, preferably,0-7%. When the content of ZnO is more than 10%, the glass tends to bedevitrified and the strain point is lowered so that the heat resistancecan not be assured.

When the total content of MgO, CaO, BaO, SrO, and ZnO is less than 5%,the high-temperature viscosity is increased so that the meltability isdegraded and the glass is readily devitrified. The total content morethan 30% is unfavorable because the heat resistance and the acidresistance are degraded.

In addition to the above-described components, it is possible to addZrO₂, TiO₂, Fe₂O₃, and the like up to 5% in total.

Next, a combination of Sb₂O₃ and at least one of SnO₂ and chloride isadded as a fining agent to the glass batch. As a material for chloride,BaCl₂, CaCl₂, and the like can be used. As a material for Sb₂O₃, apentavalent Sb compound such as Sb₂O₅ can be used instead of Sb₂O₃. Theamounts of Sb₂O₃ and SnO₂ to be added is 0.05-3 wt % and 0.05-2 wt %,respectively, with respect to the raw glass mixture as 100 wt %. On theother hand, the amount of chloride is 0.01-2 wt % in terms of Cl₂, thatis, an amount for producing 0.01-2 wt % Cl₂ by decomposition of thechloride. When the content of Sb₂O₃ is less than 0.05%, it is difficultto expel the gas generating during the batch decomposition anddegassing. When the content of SnO₂ is less than 0.05% and the chlorideis less than 0.01% in terms of Cl₂, it is difficult to remove thebubbles remaining in the glass melt during the fining and homogenizing.On the other hand, when the content of SnO₂ is more than 2% and thecontent of Sb₂O₃ is more than 3%, the glass tends to be devitrified.Furthermore, when the chloride is more than 2% in terms of Cl₂, theamount of volatilization is excessive so that the glass tends to bedeteriorated. It is possible to use only one of SnO₂ and chloride.However, the use of both of them in combination is favorable because theexcellent fining effect is obtained.

Then, the glass batch thus prepared is melted. When the glass batch isheated, at first the batch decomposition and degassing occurs. At thistime, the oxygen gas generates as a result of chemical reactionfollowing the variation in valence of Sb₂O₃. Therefore, the gasgenerating at the time of batch decomposition and degassing is expelledfrom the glass melt. Furthermore, during the fining and homogenizing ata higher temperature, the oxygen gas generates as a result of chemicalreaction following the variation in valence of SnO₂. In addition,chloride is decomposed and volatilized to produce the chlorine gas orchlorine compound gas. Thus, very small bubbles remaining in the glassmelt are removed.

Then, the glass melt is formed into a desired shape. For use as adisplay, the glass melt is formed into a thin plate shape by the use ofa fusion process, a downdraw process, a float process, a roll-outprocess, and the like.

Thus, it is possible to obtain the alkali-free glass of the presentinvention, essentially consisting of basic elements of 40-70% SiO₂,6-25% Al₂O₃, 5-20% B₂O₃, 0-10% MgO, 0-15% CaO, 0-30% BaO, 0-10% SrO, and0-10% ZnO and a combination of 0.05-3% Sb₂O₃ and at least one of 0.05-2%SnO₂ and 0.005-1% Cl₂ as the fining agent.

In the following, the alkali-free glass of the present invention will bedescribed in conjunction with specific examples.

EXAMPLE 1

Tables 1 and 2 show the effects of Sb₂O₃, SnO₂, and chloride. Herein, asample a is a conventional alkali-free glass with As₂O₃ added as afining agent. A sample b is an alkali-free glass prepared from thesample a with As₂O₃ removed therefrom. A sample c is an alkali-freeglass with only Sb₂O₃ added as a fining agent. A sample d is analkali-free glass with only SnO₂ added as a fining agent. A sample e isan alkali-free glass with only chloride (BaCl₂) added as a fining agent.A sample f is an alkali-free glass according to the present inventionwith Sb₂O₃ and SnO₂ used in combination. A sample g is an alkali-freeglass of this invention with Sb₂O₃ and chloride used in combination. Asample h is an alkali-free glass according to the present invention withSb₂O₃, SnO₂, and chloride used in combination.

In Tables 1 and 2, each of the sample glass batches consists of a totalamount of 100 wt % of glass forming ingredients of SiO₂ thorugh ZnO inthe Tables and an addition of the fining agent of Sb₂O₃ through As₂O₃alone or in combination.

TABLE 1 (wt %) Sample No. a b c d e Glass SiO₂ 60.0 60.0 60.0 60.0 60.0Batch Al₂O₃ 16.0 16.0 16.0 16.0 16.0 Composition B₂O₃ 8.5 8.5 8.5 8.58.5 MgO 4.0 4.0 4.0 4.0 4.0 CaO 1.0 1.0 1.0 1.0 1.0 BaO 6.0 6.0 6.0 6.06.0 SrO 3.5 3.5 3.5 3.5 3.5 ZnO 1.0 1.0 1.0 1.0 1.0 Sb₂O₃ — — 0.3 — —SnO₂ — — — 0.3 — Cl₂ — — — — 1.0 As₂O₃ 0.3 — — — — Fineness 1500° C. · 1hr ⊚ X Δ Δ Δ 1550° C. · 1 hr ⊚ X Δ ◯ Δ

TABLE 2 (wt %) Sample No. f g h Glass SiO₂ 60.0 60.0 60.0 Batch Al₂O₃16.0 16.0 16.0 Composition B₂O₃ 8.5 8.5 8.5 MgO 4.0 4.0 4.0 CaO 1.0 1.01.0 BaO 6.0 6.0 6.0 SrO 3.5 3.5 3.5 ZnO 1.0 1.0 1.0 Sb₂O₃ 0.3 0.3 0.3SnO₂ 0.3 — 0.3 Cl₂ — 1.0 1.0 As₂O₃ — — — Fineness 1500° C. · 1 hr ◯ ◯ ⊚1550° C. · 1 hr ◯ ◯ ⊚

Each of the samples was prepared as follows.

Glass materials were mixed to obtain a glass batch having a compositionspecified in Tables. The glass batch was melted in an electric furnaceto obtain glass melt. At this time, two types of glass melt wereprepared one of which was melted at 1500° C. for 1 hour in order toevaluate the fineness or fining characteristic during the batchdecomposition and degassing and the other of which was melted at 1550°C. for 1 hour in order to evaluate the fineness during fining andhomogenizing. Next, the glass melt was poured on a carbon table andslowly cooled. Thereafter, the number of bubbles remaining in the glasswere counted so as to determine the fineness. Here, the fineness means ameasure which shows a degree of numbers of bubbles remaining in theglass as the results of the fining action or effect by the finingagents. The fineness is classified into four ranks, that is “bad”, “notgood”, “good” and “very good”, according to the number of bubbles. Theranks were shown in the tables by symbols X, Δ, ◯, and ⊚ representingthe cases where the number of the bubbles in the glass of 100 g is morethan 1000, 101-1000, 11-100, and 10 or less, respectively. In thecomposition of the glass batch shown in these Tables, the content ofeach component is represented in terms of oxide except that the contentof chloride is represented in terms of Cl₂.

As is obvious from Tables 1 and 2, the sample b with no fining agentadded seriously degrades in fineness.

The sample c with only Sb₂O₃ added generated a large amount of fininggas during the batch decomposition and degassing. However, a sufficientamount of fining gas was not generated during fining and homogenizing.As a result, the fineness was degraded.

The sample d with only SnO₂ added generated a large amount of fining gasduring fining and homogenizing. However, the fineness was insufficientduring batch decomposition and degassing. As a result, the fineness wasinferior as compared with the sample a with As₂O₃ added.

Similarly, the sample e with only chloride added generated a largeamount of fining gas during fining and homogenizing. However, asufficient amount of fining gas was not generated during batchdecomposition and degassing. As a result, the fineness was degraded. Onthe other hand, the fineness was excellent in each of the samples f to hwith Sb₂O₃, SnO₂, and/or chloride added.

EXAMPLE 2

Tables 3 to 6 show examples (Samples Nos. 1 to 20) of the alkali-freeglass obtained according to the present invention. The amount of each ofingredients is represented by the weight percent in each sample glassproduced.

TABLE 3 (wt %) Sample No. 1 2 3 4 5 Glass SiO₂ 54.0 56.3 58.7 62.3 64.4Composition Al₂O₃ 19.6 10.7 16.5 17.5 19.5 B₂O₃ 10.5 8.4 8.3 8.5 5.5 MgO— — 3.7 4.5 0.3 CaO 3.1 5.4 1.0 — 5.9 BaO 1.8 13.0 5.8 1.1 0.3 SrO 8.94.2 3.1 0.6 0.6 ZnO — 1.3 0.9 2.7 — Sb₂O₃ 0.9 0.4 1.2 1.3 1.7 SnO₂ 1.20.3 0.8 1.5 1.8 Cl₂ — — — — — Fineness 1500° C. · 1 hr ◯ ◯ ◯ ◯ ◯ 1550°C. · 1 hr ◯ ◯ ◯ ◯ ◯ Strain Point (° C.) 677 628 662 671 710Hydrochloric-acid ◯ ◯ ◯ ◯ ◯ Resistance Buffered-hydrofluoric- ◯ ◯ ◯ ◯ ◯acid Resistance

TABLE 4 (wt %) Sample No. 6 7 8 9 10 Glass SiO₂ 58.4 56.2 54.6 59.3 62.4Composition Al₂O₃ 16.5 11.0 19.9 16.4 17.8 B₂O₃ 9.0 8.3 10.6 8.5 8.4 MgO— — — 3.9 4.7 CaO 2.1 5.4 3.0 0.8 — BaO 3.5 13.2 2.0 5.9 1.3 SrO 6.5 4.09.0 3.0 0.8 ZnO 0.5 1.5 — 1.0 3.1 Sb₂O₃ 2.5 0.3 0.6 1.0 1.1 SnO₂ 1.0 — —— — Cl₂ — 0.06 0.3 0.2 0.4 Fineness 1500° C. · 1 hr ◯ ◯ ◯ ◯ ◯ 1550° C. ·1 hr ◯ ◯ ◯ ◯ ◯ Strain Point (° C.) 665 625 679 665 669 Hydrochloric-acid◯ ◯ ◯ ◯ ◯ Resistance Buffered-hydrofluoric- ◯ ◯ ◯ ◯ ◯ acid Resistance

TABLE 5 (wt %) Sample No. 11 12 13 14 15 Glass SiO₂ 65.3 57.7 48.0 56.059.0 Composition Al₂O₃ 19.8 15.7 11.0 10.5 15.0 B₂O₃ 5.6 8.5 14.5 5.510.5 MgO 0.3 3.9 — 2.0 0.5 CaO 6.2 0.8 — 3.5 4.5 BaO 0.4 6.1 25.0 15.06.0 SrO 0.5 3.3 — 6.0 3.0 ZnO — 1.1 — — — Sb₂O₃ 1.4 2.4 0.5 0.3 0.9 SnO₂— — 0.5 0.9 0.3 Cl₂ 0.5 0.5 0.5 0.3 0.3 Fineness 1500° C. · 1 hr ◯ ◯ ⊚ ⊚⊚ 1550° C. · 1 hr ◯ ◯ ⊚ ⊚ ⊚ Strain Point (° C.) 712 655 595 640 645Hydrochloric-acid ◯ ◯ ◯ ◯ ◯ Resistance Buffered-hydrofluoric- ◯ ◯ ◯ ◯ ◯acid Resistance

TABLE 6 (wt %) Sample No. 16 17 18 19 20 Glass SiO₂ 58.0 63.0 66.0 54.064.5 Composition Al₂O₃ 16.0 18.0 19.5 19.5 19.0 B₂O₃ 8.5 8.0 5.5 10.56.0 MgO 1.0 5.0 — — 0.5 CaO 4.0 — 6.5 3.0 6.0 BaO 9.5 1.0 0.5 2.0 0.5SrO 2.0 1.0 0.5 8.5 0.5 ZnO — 3.0 0.5 — — Sb₂O₃ 0.3 0.6 0.3 1.2 2.3 SnO₂0.5 0.3 0.6 1.2 0.5 Cl₂ 0.2 0.1 0.1 0.1 0.2 Fineness 1500° C. · 1 hr ⊚ ⊚⊚ ⊚ ⊚ 1550° C. · 1 hr ⊚ ⊚ ⊚ ⊚ ⊚ Strain Point (° C.) 660 670 710 675 700Hydrochloric-acid ◯ ◯ ◯ ◯ ◯ Resistance Buffered-hydrofluoric- ◯ ◯ ◯ ◯ ◯acid Resistance

In the composition of each of the glasses shown in these Tables, thecontent of each component is represented in terms of oxide and exceptthat the amount of chloride remaining in the glass is represented interms of Cl₂.

Each of the samples was prepared as follows.

Glass materials were mixed to obtain a glass composition specified inthese Tables. BaCl₂ was used as chloride. Then, a glass plate wasprepared therefrom and subjected to evaluation of the fineness in themanner similar to Example 1. The result is shown in Tables 3-6. Each ofthe glass sample plates is seen from the tables to be excellent, thatis, “good” or “very good” in the fineness.

On the other hand, the glass batch was also melted in an electricfurnace at 1550-1600° C. for 16-24 hours to obtain glass melt. The glassmelt was formed into a glass sample plate. The glass sample plate thusobtained was evaluated for heat resistance and chemical resistance. Theresult is shown in Tables 3 through 6.

As is obvious from Tables, each of the glass sample plates was excellentin the heat resistance and chemical resistance.

For evaluating the heat resistance, the strain point was measured by theuse of the ASTM C336-71 method. The chemical resistance was evaluatedfor the hydrochloric-acid resistance. Specifically, after each sampleplate was immersed for 24 hours in a 10 wt % hydrochloric acid solutionheld at 80° C., the surface condition of the glass sample plate wasobserved. The symbol X represents occurrence of discoloration while thesymbol ◯ represents no discoloration. On the other hand, thebuffered-hydrofluoric-acid resistance was evaluated in the followingmanner. Specifically, after each sample plate was immersed for 30minutes in a buffered hydrofluoric acid solution held at 20° C. andcomprising 38.7 wt % ammonium fluoride and 1.6 wt % hydrofluoric acid,the surface condition of the glass sample plate was observed. The symbolX represents occurrence of cloudness on the surface of the glass sampleplate while the symbol ◯ represents no change.

As described above, according to the present invention, it is possibleto produce the alkali-free glass which has an excellent fineness and isfree from bubbles resulting in the display defect, by the use of acombination of Sb₂O₃ and at least one of SnO₂ and chloride as the finingagent.

Furthermore, the alkali-free glass according to the present invention isfree from bubbles resulting in the display defect and excellent in heatresistance and chemical resistance. Therefore, the alkali-free glass isparticularly suitable as a transparent glass substrate for a liquidcrystal display.

What is claimed is:
 1. A method of producing an alkali-free glass havinga basic composition consisting essentially, by weight, of 40-70% SiO₂,6-25% Al₂O₃, 5-20% B₂O₃, 0-10% MgO, 4.5-15% CaO, 0-30% BaO, 0-10% SrO,and 0-10% ZnO, and containing substantially no alkali metal oxide, saidmethod comprising the steps of preparing a glass batch of the basiccomposition, including an addition of a fining agent consisting of acombination of 0.05-3% Sb₂O₃, and at least one of 0.05-2% SnO₂ and0.01-2 wt % chloride in terms of Cl₂, melting said glass batch into aglass melt, and forming said glass melt into a desired shape.
 2. Amethod of producing an alkali-free glass as claimed in claim 1,saidalkali-free glass being for use as a transparent glass substrate for aliquid crystal display.
 3. A method of producing an alkali-free glass asclaimed in claim 1, wherein said fining agent consists of 0.05-3% Sb₂O₃,0.05-2% SnO₂, and 0.01-2 wt % chloride in terms of Cl₂.
 4. A method ofproducing an alkali-free glass as claimed in claim 1, wherein saidfining agent consists of 0.05-2% Sb₂O₃ and 0.05-2% SnO₂.
 5. A method ofproducing an alkali-free glass as claimed in claim 1, wherein saidfining agent consists of 0.05-3% Sb₂O₃ and 0.01-2 wt % chloride in termsof Cl₂.